Measurement of light absorption by chromophoric dissolved organic matter using a type-II liquid capillary waveguide: assessment of an achievable accuracy.

Light absorption by chromophoric dissolved organic matter (CDOM) in the ocean is often measured using liquid waveguide capillary cells coupled to spectral array detectors. This type of optical setup is affected by several sources of uncertainties related to the waveguide and the detector. Uncertainties from the waveguide arise from errors in the effective path length and the effects of water salinity, while errors related to the detector are due to the non-linearity in the response, internal stray light, and wavelength accuracy. Here, uncertainties in the measurements of the spectral absorption coefficient of CDOM due to the optical setup itself were investigated in detail. The related systematic errors were very often significant (2-15%) and larger than expected from simple measurement uncertainty (±1%). However, they can be corrected by characterizing the detector's response for non-linearity and stray light, regularly performing calibrations for the detector's wavelength response, and routinely measuring the waveguide's effective path length. Including such corrections and timely calibrations reduces the uncertainties related to the spectrophotometric measurements to about ±2%. Uncertainties related to the necessary handling of samples are not included here.

Characterization of a fast response fiber-optic pH sensor and illustration in a biological application.

Optical, and especially fiber-optic techniques for the sensing of pH have become very attractive and considerable research progress in this field has been made over recent years. The determination of the value of pH across a broad range of applications today, important for areas of study such as life sciences, environmental monitoring, manufacturing industry and widely in biological research is now accessible from such optical sensors. The need for such technology arises because familiar, commercial sensors are often limited in terms of their response time and the presence of drift, all of which emphasize the value of newer and rapidly developing technologies such as fiber-optic sensors, to address these wider applications. As a result, a new compact sensor design has been developed, designed around a specially-formed fiber-optic tip, coated with a pH-sensitive dye, and importantly covalently linked to a hydrogel matrix to provide high stability. The sensor developed was designed to have a very fast response time (to 90% of saturation, Δt90) of <5 s and a sensing uncertainty of ∼±0.04 pH units. Given the covalently bonded nature of the dye, the problem of leaching of the indicator dye is reduced, creating a probe which has been shown to be very stable over many days of use. Illustrating this through extended continuous use, over ∼12 h at pH 7, this stability was confirmed showing a drift of <0.05 pH h-1. In order to give an illustration of the value of the probe in an important biological application, the monitoring of pH levels between pH 7 to pH 8 in an AMES' medium, a substance which is important to maintain the metabolism of retinal cells is shown and the results as well as temperature stability of the probe discussed.